1. Field of the Invention
The present invention relates to a loop type continuous metal casting machine and, more particularly, to a twin-belt casting machine of the above type, in which two side dams are revolved in a loop passing along a casting zone from the entrance end thereof between upper and lower revolving casting belts to define a downhill moving mold and in which each of the side dams is formed of a multiplicity of metal dam blocks so strung onto a flexible metal strap (or wire) loop that the metal dam blocks abut in end-to-end relationship against one another but are allowed to slide on and relative to the metal strap (or wire) loop.
2. Description of the Prior Art
In such casting machines, the two side surfaces of the mold region are defined by a pair of spaced side dams which are divided into two types: the stationary and moving types. In order to eliminate a variety of bad influences which are caused by the endothermic actions and thermal deformation of the side dams while the cast metal is being solidified and extracted, the stationary or fixed dam type casting machines have to adopt a water-cooling apparatus and to take countermeasures for preventing any possible sticking of cast metal on the dam surface so that their constructions become complicated. Since the cast metal slides on the surfaces of the fixed side dams, moreover, these side dams tend to wear off. From the standpoint of quality, on the other hand, the side surfaces of the cast metal are degraded as a result of the sticking, and the degraded side surfaces are doubled to raise defects during a subsequent rolling operation because the molten metal will steal into those gaps between the side surfaces of the metal and the side surfaces of the side dams, which are formed as a result of the solodification and shrinkage. These defects on side surfaces of the cast metal have to be cured at a subsequent step added. From the reasonings made above, the fixed dam type casting machines are not used at present for longtime casting operations.
It is the moving dam type casting machines that are devised to solve the aforementioned problems of the fixed dams type casting machines. Therefore, the moving dam type casting machines are currently used for the long casting operations.
These moving dam type casting machines are exemplified by a twin-belt casting machine which will be described with reference to FIGS. 1 to 6. This casting machine, which is generally indicated at numeral 10, is constructed to include upper and lower endless casting belts 11 and 12 which are spaced from each other and which are revolved by two pairs of rolls 13 and 14, respectively. A moving mold has its upper and lower surfaces defined by the paired casting belts 11 and 12. The two side surfaces of the moving mold are defined by a pair of two side dams 15, each of which is composed of a multiplicity of metal damblocks 16. The side dams 15 are revolved in the form of a loop, which passes along a casting zone a from the entrance end thereof, between the revolving casting belts 11 and 12 by the lower belt 12 to define a downhill moving mold between the side dams 15. The side dam loop returns from the exit end to the entrance end of the casting zone a along a path b which is located away from the casting zone a. The metal damblocks 16 are slotted therethrough so that they can be strung onto each of two flexible metal strap loops 17, as better seen from FIG. 2. As a result, a pair of side dam loops are formed, in which the metal damblocks 16 abut in end-to-end relationship against one another but are allowed to slide on and relative to the metal strap loops 17.
Here, the side dam loops are ordinarily moved not by a special driving apparatus but by the structure in which they are driven by the frictional forces generated as a result of their contacts with the lower casting belt 12 when the upper and lower belts 11 and 12 are revolved by the rolls 13 and 14. During the travel, the side dams 15 are heated by the cast metal being cast so that their temperature gradually rises. Turning to FIG. 3, therefore, there is disposed below the lower casting belt 12, i.e., downstream of the exit end of the casting zone a a cooling apparatus 18 which prevents the temperature of the traveling side dams 15 from rising over a predetermined level.
In FIGS. 3, 4 and 5: reference numeral 19 indicates a dam side guide; numeral 21 a plurality of entrance end guide rollers; and numerals 22 and 23 front and rear flanged rollers, all of which are used, as customary, to guide and regulate together the moving side dams 15.
The moving dams type continuous metal casting machine of the prior art has the following problem:
A first problem, i.e., the problem intrinsic to the moving side dams 15 is that, since each side dam 15 is given an allowance for thermal expansion about one thousandth as large as its total loop length, the gaps are either accumulated to as large as 5 to 10 mm so as to prevent the steel strap loops 17 from being broken, in case the allowance is concentrated at one portion, or are scattered at several portions. This will be described in more detail in the following. Gaps are formed in advance between the damblocks 16 because the steel strap loops 17 and the damblocks 16 strung thereon exert different expansions. In the ordinary run, as better seen from FIG. 1, those gaps have a tendency to concentrate at or around a gap zone c which is located downstream of the exit end of the casting zone a, i.e., downstream of the righthand lower roll 14. As the slippages between the respective damblocks 16 and the steel strap loops 17 grow worse, however, the gaps are frequently formed even in the casting zone a. Then, the molten metal in the moving mold leaks into the gaps between the damblocks 16 in the casting zone a to produce irregular sides or burrs on the cast product. Another but more serious problem is that the molten metal flows out to invite dangers, if the stealing rate of the molten metal is so high that the molten metal damages and breaks the exposed portion or portions of the steel strap loops 17. And, if the molten metal steals into the gaps and solidifies therein, moreover, the cast product is pulled and broken at the exit end of the casting machine by the damblocks 16 to make the casting operation impossible. In order to avoid this problem, therefore, it is the solution according to the prior art that the damblocks at the entrance end of the casting machine are manually pushed until the gaps disappear due to thermal expansion. However, this solution raises another problem in safety.
In order to eliminate these difficulties, as shown in FIGS. 2 and 6, the steel strap loop 17 is positioned toward the interior of the side dam loop with respect to the longitudinal centerline of each damblock, and a tensioning apparatus 24 is located at the return side of the side dam loop to push and deflect upward a portion of the depending side dam loop thereby to eliminate the slackness among the damblocks 16 in the casting zone a through a downstream zone d and d' which extends from the exit to the entrance end of the casting zone a, as better seen from FIG. 6 (as should be referred to Japanese Patent Publication No. 58-23181). Despite of this fact, however, the total amount of the compensational gaps g.sub.1, g.sub.2, . . . , g.sub.5 in zone e given by that tensioning apparatus 24 is so limitative that the initial gaps formed during the thermal expansions cannot be absorbed sufficiently. Even if this absorption can be achieved, it is quite difficult to adjust the deflections at all times during the actual operation in accordance with the amount of thermal expansions.
Therefore, it can be said that there is no effective means for preventing the excessively large gaps formed between the damblocks from being accumulated in the casting zone. Thus, it is the current practice to push the damblocks one by one by the operator's hands. This manual practice invites a serious danger in handling the hot molten metal and degrades the rate of operation.
A second problem is concerned with the drive of the moving side dams 15. Because of shortage of any driving apparatus for damblocks themselves, as has been touched hereinbefore, each side dam 15 may be halted to behave as the fixed one even if it is slightly dragged by its loop. This makes it necessary to eliminate causes for the frictional resistances as much as possible between the moving side dam 15 and a guide liner 25 which lines the inner side of each dam side guide 19, as seen from FIG. 5. For this necessity, it is a current practice to apply the Si-oil or a graphite coating as the anti-seizure agent to the guide liner 25 or to increase the hardness of the liner material.
In case the molten metal is to be cast, the cast slab may be caused to meander by the irregular cooling or by the inward deflection of the side dam 15 while it is being solidified and shrunk in the moving mold. In case a transverse pushing force f is exerted upon the moving side dam 15, as shown in FIG. 5, this side dam 15 tends to be forced onto the guide liner 25 of the dam side guide 19 so that either the guide liner 25 or the moving side dam 15 is scratched. If these scratches gradually grow, moreover, the moving side dam 15 may be halted or may be wedged between the guide liner 25 and the metal being cast to bite the side surface of the cast slab. At the same time, the steel strap loop 17 may be accidentally broken so that the casting operation has to be interrupted at last.
In order to eliminate these difficulties, there have been devised: a method of increasing the frictional resistance between the upper and lower casting belts 11 and 12 and the moving side dams 15 as high as possible; a method of either knurling or shot blasting the outer surface of the lower casting belt 12 or knurling the bottom surfaces of the side dams 15 so that they may come into much friction; and a method of increasing the hardness or lubrication of the contacting sides of the side dams 15 with the guide liners 25. However, none of the above-enumerated methods have succeeded in satisfying the intended purposes. Since the side dams 15 are driven by the friction force between the lower casting belt 12 and the side dams 15 themselves, on the other hand, it is conceivable to hold and move the side dams 15 by both the upper and lower casting belts 11 and 12. However, these belts 11 and 12 are heated to expand by the molten metal being gradually cast to solidify so that they are dented or scratched to have their lives shortened.
Another method of smoothening the travel of each side dam 15 is to enlarge the net weight of the side dam 15 thereby to increase the frictional force which is applied to the side dam 15 from the lower casting belt 12. However, this method is practically impossible because of the restrictions on the actual works such as the handling works of mounting or demounting the heavy side dam 15.
It is, therefore, necessary to make the best use of the effective weight of the side dam 15. In the prior art, however, the weight of the side dam 15 is borne, as better seen from FIG. 4, by the front and rear flanged rollers 22 and 23 which are borne in horizontal positions on the front and rear portions of the cooling apparatus 18, as shown in FIG. 3. This positioning makes it impossible to effectively use the weight of the side dam 15. This is because the weight of the side dam 15 in the casting operation is borne by the front and rear rollers 22 and 23 in accordance with the prior art, as shown in FIG. 4.
On the other hand, the method, as disclosed in Japanese Patent Publication No. 58-23181, is acceptable for eliminating the gaps of the moving damblocks 16. However, this method will cause obstructions to the travels of the side dam loops.